The Importance of Expansion Tanks for Water Heaters

Learn about the necessity of expansion tanks for water heaters and their benefits and drawbacks. Find out how to determine the right size and test for any issues.

The Importance of Expansion Tanks for Water Heaters

As a plumbing expert, I have seen firsthand the consequences of not having an expansion tank for a water heater. It may seem like a small and insignificant component, but it plays a crucial role in maintaining the safety and efficiency of your plumbing system. In fact, expansion tanks are required by code on all new installations of water heaters in closed systems. But what exactly is an expansion tank and why is it necessary? Let's dive into the details.

What is an Expansion Tank?

An expansion tank is a small overflow tank that compensates for the thermal expansion of water inside a water heater. It is typically installed next to the water heater and is connected to the cold water supply line.

When water is heated, it expands and increases in pressure. In a closed system, this pressure can cause damage to plumbing fittings and pipes, or even the water heater itself. The expansion tank acts as a safety valve, absorbing the excess pressure and preventing any potential damage.

Why is it Required by Code?

The need for expansion tanks stems from the use of check valves, backflow prevention devices, pressure reducing valves, or other devices in the supply line to prevent water from returning to the municipal water supply. These regulations were put in place by the EPA in 1992 to protect municipal water pipes from reflux contamination.

As a result, homes with these devices are considered closed circuit systems and must have an expansion tank installed with their water heater. Furthermore, state and federal laws and codes require that water heaters have temperature and pressure (T&P) relief valves for personal safety. These valves release excess pressure from the tank if it reaches dangerous levels. However, without an expansion tank, the T&P valve may constantly release small amounts of water, causing unnecessary wear and tear on the valve and wasting water.

How to Determine the Right Size

The size of your expansion tank should be based on the capacity of your water heater and the water pressure in your home. As a general rule, a 2-gallon expansion tank is suitable for standard water heaters up to 50 gallons in capacity, as long as the static supply is less than 60 pounds of force per square inch (PSI).

If you have a larger water heater or higher water pressure, you may need a larger expansion tank.

The Advantages and Disadvantages

Now that we understand what an expansion tank is and why it is required, let's take a look at its advantages and disadvantages.


  • Prevents damage to plumbing fittings and pipes
  • Protects the water heater from excess pressure
  • Reduces wear and tear on T&P valves
  • Saves water by preventing constant release from T&P valves


  • Additional cost for installation
  • Takes up space next to the water heater
While there are some drawbacks to having an expansion tank, the benefits far outweigh them. Not only does it protect your plumbing system, but it also saves you money in the long run by reducing potential damage and water waste.

How to Test for a Faulty Expansion Tank

If you already have an expansion tank installed, it's important to regularly check for any issues. A faulty expansion tank can cause problems such as low water pressure or inconsistent hot water temperature. To test for a faulty expansion tank, follow these steps:
  1. Turn off the water supply to the water heater
  2. Open a hot water faucet in your home to release any pressure
  3. Check the air pressure in the expansion tank using a tire pressure gauge
  4. If the pressure is significantly lower than the recommended PSI, the tank may need to be replaced
If you are unsure about how to test your expansion tank, it's best to consult a professional plumber.

In Conclusion

In summary, an expansion tank is a necessary component for any closed plumbing system.

It protects your plumbing system and water heater from excess pressure and potential damage. While it may require an additional cost and take up some space, the benefits far outweigh any drawbacks. If you are installing a new water heater or have a closed plumbing system without an expansion tank, I highly recommend adding one for the safety and efficiency of your home.

Randy Strombach
Randy Strombach

Plumbing Contractor, father to three great kids and husband to an amazing wife